Petrochemical products such as oil and gas, are ubiquitous in society and can be found in everything from gasoline to children's toys. Because of this, the demand for oil and gas remains high. In order to meet this high demand, it is important to locate oil and gas reserves in the Earth. Scientists and engineers conduct “surveys” utilizing, among other things, seismic and other wave exploration techniques to find oil and gas reservoirs within the Earth. These seismic exploration techniques often include controlling the emission of seismic energy into the Earth with a seismic source of energy (e.g., dynamite, air guns, vibrators, etc.), and monitoring the Earth's response to the seismic source with a receiver (e.g., a geophone, a hydrophone, etc.). By observing the reflected seismic signals detected by the receiver during the survey, the geophysical data pertaining to reflected signals may be acquired and these signals may indicate the composition of the Earth proximate the survey location.
As portions of the oil and gas are removed from the reservoir, a follow-up, or monitor survey may be taken near the same location as the original, baseline survey. Often-times, the data that the monitor survey provides is called 4D data, because it provides a fourth dimension to the data being collected—time. For example, this 4D survey may allow scientists and engineers to see how much oil or gas has been removed from the reservoir, how much oil or gas remains in the reservoir, and/or how the oil or gas has moved within the reservoir since the baseline survey. In cases where the sources and receivers for the baseline survey are installed on land or on the surface of the ocean floor and not removed between surveys, the monitor survey may involve firing the sources and recording the reflected seismic signals at the receivers. In other cases, such as where the surveys are conducted using streamers towed behind a vessel, or using transient sources and receivers on land, some system of quality control may be established to facilitate coordinating data sets between the two surveys.
“Repeatability” is a term used to refer to the measure of how closely the monitor survey correlates to the baseline survey. There are several different forms of repeatability. For example, positional or geometric repeatability is a measure of how positionally or geometrically close a monitor survey's data is to a baseline survey's data. Even if the geometric repeatability of a monitor survey is good (for example, where sources and receivers are installed permanently on land or on the ocean floor), other factors may decrease the repeatability, and therefore the reliability, of the data obtained in the monitor survey. A few aspects of a survey that may be of concern in terms of repeatability include: source position, receiver position, the common mid-point (CMP) position between the source and the receiver, offset or radial distance between the source and the receiver, azimuth, and components and combinations of these. In order to accurately determine the amount of oil or gas removed from a reservoir, scientists and engineers may need to know how closely the monitor survey correlates to one or more aspects of the baseline survey.